There have been proposed fixing devices of various systems to heat-fixing unfixed toner images in an image forming apparatus such as an electrophotographic copying machine, printer, or facsimile. Such fixing devices include, for example, a fixing device of a belt-nip system in which a fixing belt is pressed against a heating member or a pressing member.
There is cited, for example, a fixing device of a belt-nip system, as disclosed in JP 2004-109878A, which is constituted of a rotatably supported heating member, an endless belt with its outer circumference pressed against the heating member, the inner circumference of the endless belt and a pressure-applying member, pressed against the inner circumference of the endless belt and pressing the heating member through the endless belt along the outer circumference of the heating member. Such a fixing device of a belt-nip system can maintain a sufficient nip-transit time, that is, a sufficient contact time of an unfixed toner image with a heating member, so that an image of high gloss can be obtained.
However, such a fixing device of a belt-nip system, in which a pressure-applying member with an upper surface formed of an elastic layer such as rubber is disposed with being brought into contact with the inner circumference of an endless belt, results in increased slide resistance, leading to unstable rotary movement and producing problems such as occurrence of slippage of images. Specifically in cases when forming a solid image with an increased toner adhesion amount, occurrence of slippage of images results in microscopic wrinkles, which are visibly observed as uneven gloss.
Accordingly, to solve such problems is disclosed a technique of covering the upper surface of a pressure-applying member (hereinafter, also denoted as a slide-sheet) with a sheet member of a glass fiber sheet coated with a fluororesin (PFA) to inhibit abrasion of the inner circumference surface of a endless belt and a technique of covering the surface of a pressure-applying member with a slide sheet of a concave-convex surface to lessen contact areas to reduce friction, as described in JP 2002-148970A. However, such techniques complicate the constitution of a fixing device, producing problems such that an exchange cycle is shortened along with deterioration of the belts or slide sheets.